This invention relates generally to an improved electron gun system for television receiver cathode ray tubes that provides at least partIal dynamic beam convergence substantially independently of any beam-focus-related adjustments in the main focusing field, and without introducing significant beam distortion. The invention has applicability to all types of color picture tubes and to all types of beam convergence systems including those dependent on the selfconverging yoke and the uniform field yoke. With regard to gun systems, the invention has application to the many types used in home-entertainment television systems and computer display monitors. It also may be advantageously applied to systems that utilize an extended field main focus lens. The dynamically converging gun system according to the invention is particularly usefu1 in improving the image resolution of flat-faced cathode ray tubes which utilize the tension foil mask, and in which degradation of screen corner resolution and edge resolution is particularly troublesome.
Desired picture tube performance characteristics of color television receiver systems include high resolution, picture brightness, and color purity. Resolution is largely a function of the size and symmetry of the beam spots projected by the electron guns of the picture tube. Beam spots are desirably small, round, and uniform in size at all points on the picture screen. Achievement of these ideals is difficult because of the many factors which exert an influence on the configuration of beam spots. As a result of such factors, a beam spot that is small and symmetrical at the center point of the picture imaging field can become enlarged and distorted at the periphery of the field, for reasons which will be described.
Key factors which influence beam spot size, uniformity and symmetry in picture tubes having three-beam electron guns include the following:
(a) electron gun design; PA1 (b) cathode ray tube screen potential; PA1 (c) magnitude of beam current; PA1 (d) the "throw" distance from the electron gun to the screen; and, PA1 (e) the convergence system.
The ability of an electron gun to form small, symmetrical beam spots is a major factor in achieving optimum resolution. The task of designing guns with this capability has become more challenging because of reduction in diameter in the CRT neck. This physical constraint has been largely overcome by new, more effective gun designs, such as the gun having an extended field main focus lens described and claimed in U.S. Pat. No. 3,995,194 assigned to the assignee of this invention.
Convergence of the three beams of an in-line electron gun is provided in present-day television systems primarily by the self-converging yoke. This type of yoke is a hybrid having toroidal-type vertical deflection coils and saddle-type horizontal deflection coils. The yoke contains windings which produce an astigmatic field component that has the effect of maintaining the beams in convergence as they are swept across the screen. An example of a beam-deflecting yoke that provides for self-converging of multiple beams is disclosed in U.S. Pat. No. 3,643,102 to Chiodi. This concept has found wide application in cathode ray display tubes intended for consumer products.
The converging effect is shown highly schematically in FIG. 1, in which an electron gun 10 is depicted graphically as emitting three beams 12, 13 and 14 which diverge from a common plane 16 to impinge on a curved screen 18. The three beams are shown as being converged at the center point 20 of the screen 18. Due to the effect of the self-converging yoke, the three beams are also caused to be in convergence at the side of the screen 18, as indicated by point 22, even though the distance that beams must travel from the plant of deflection 16 to point 22 is greater than from the plane of defection 16 to center point 20 of the screen.
The convergence achieved is not without cost, however, as the beam spots are subject to distortion in the peripheral areas of the screen, as will be shown with reference to FIG. 3. The distortion is acceptable in tubes in which lower resolution is acceptable as the benefits and costs savings implicit in the self-converging yoke outweigh its liabilities.
However, when the screen is flat, as indicated by screen 24 in FIG. 2, the conventional self-converging yoke is unable to maintain beam convergence, as indicated by the spread of the beam spots 28 at the sides 26 of screen 24. In addition to the spread, the spots 28 will be noted as being elongated. This elongation is due primarily to the self-converging feature of the yoke.
The astigmatic field component, while self-converging the beams, undesirably introduces an astigmatic deflection defocusing of the beams when the beams are deflected away from the screen center point. This effect is indicated diagrammatically in FIG. 3 by beam spots 34. The elongation of the beam spots at the peripheries of the faceplate, and the relative increase in spot size, is indicated in greater detail in the inset figure. FIG. 3A. The beam spots 34 will be seen as comprising a bright core 34A, and transverse to the core, a dim "halo," 34B. The center beam spot 36C is shown to illustrate the magnitude of the spot size increase and distortion at the screen corner. Attempts to focus such beams are largely ineffectual due to the astigmatic effect--focusing merely resu1ts in what appears to be a "rotation" of the spot in that the core becomes the halo and the halo becomes the core.
As has been noted, the effect is tolerable in conventional tubes where the screen is curved, as shown by FIG. 1, and it is acceptably withn the capability of the self-converging yoke to converge the beams without undue distortion. However, when the screen is flat, as indicated by FIG. 2, the astigmatic effect of the self-converging yoke is no longer tolerable, especially in high-resolution cathode ray tubes. Any attempt to further modify the configuration of the self-converging yoke field to adapt it to a flat screen will inevitably increase distortion outside the limits of acceptability. The self-converging ability of the yoke was already stretched to its limit in its use with the curved screen before the advent of the flat tension mask tube.
Prior art structures for converging electron beams have relied upon a variety of techniques such as the use of magnetic influences within and/or without the tube envelope, and the use of electrostatically charged plates Also, the prior art shows many examples of inducing beam divergence or convergence by inducing an asymmetry in an electrostatic field formed at the interface of the two spaced electrodes. An example of this approach is found in In U.S. Pat. No. 4,058,753, where there is disclosed a three-beam electron gun for color cathode ray tube having an extended field main focus lens. The focus lens means has for each beam at least three electrodes including a focus electrode for receiving a variab1e potential for electrically adjusting the focus of the beam. In succession down-beam, there are at least two associated eectrodes having potentials thereon which form in the gaps between adjacent electrodes significant main focus field components. To adjust beam focus, the strength of a first of these components is controlled by adjustment of the voltage received by the focus electrode. The strength of the second of the field components is relatively less than that of the first component. Each of the lens means is characterized by having addressing faces of the associated electrodes which define the second field component being so structured and disposed as to cause the second field component to be asymmetrical and effective to significantly divert the beam from its path in convergence of the beams without any significant distortion of the beam and substantially independently of any beam-focusing adjustments of the first field component. Electrode structures defined for producing asymmetric fie1d components include a gap angled forwardly and outwardly, a wedge-shaped gap, and radially offset apertures.
Beam convergence in delta guns can also be obtained by means of electromagnets positioned 120 degrees apart azimuthally around the tube neck near the beam-emission points of the guns. The fields of the electromagnets are designed to aid or oppose the fields of associated permanent magnet pole pieces used for positioning the beams during set up. The beams can be dynamically converged by the application of voltages to the electromagnets which are modulated at the scanning rates. An example of such convergence means is disclosed in U.S. Pat. No. 3,379,923.
Dynamic convergence is obtained in the electron gun disclosed in U.S. Pat. No. 3,448,316 by adjustment of field potentials at scanning rates. Three in-line electron beams generated by three cathodes cross over in the electrostatic field of a main lens. The center beam (green) follows a straight-line path, but the two outer red and blue beams exit the lens in divergent paths. The outer beams pass through convergence plates which lie parallel to the gun axis and are suspended from the end of the gun nearest the screen. The potential on two outer plates is adjustable to provide for static convergence of the red and blue beams at the aperture mask. The center beam is unaffected as the potential on two inner plates through which it passes is left unchanged. Dynamic convergence is attained by changing the convergence control voltage on the outer two plates at the horizontal scanning frequency. The waveform of the voltage is in the form of a parabola.
In U.S. Pat. No. 4,520,292, von Hekken et al disclose means formed in the screen grid of an electron gun for urging the outer two beams of a three-beam electron gun into convergence with the center beam. The screen grid configuration includes a transversely disposed recessed portion having a substantially rectangular central portion and substantially triangular end parts. The total effect is to the tilt the field lines within the recessed portion so that the outer beams converge.
Other representative disclosures having electrode structures that influence beam convergence includes:
U.S. Pat. No. 3,952,224 to Evans
U.S. Pat. No. 3,772,551 to Hughes
U.S. Pat. No. 4,473,775 to Hosokoshi et al
U.S. Pat. No. 4,513,222 to Chen
As has been noted, convergence of the beams of a multiple-beam electron gun will vary as the beams arcuately scan the substantially planiform faceplate. Beam convergence may be achieved dynamically by slightly varying the relative angles of the beams while scanning. In dynamic convergence by circuit means, signals to induce dynamic convergence may be derived from the horizontal and vertical circuits of the television receiver system or monitor to provide a dynamic convergence-correction signal having the characteristlcs of a parabola. The voltage of the convergence-correcting signal is zero at the center of the picture imaging field, and changes towards the sides of the screen to effect convergence. Such dynamic convergence signals may be applied to convergence coils located adjacent to the picture tube neck, or to convergence plates suspended from the end of the gun. Such a dynamic convergence circuit is disclosed by Nelson in U.S. Pat. No. 2,834,911 in which parabolic convergence current waves are obtained by integration of pulse and saw tooth voltage waves in resistive and inductive reactive circuits, acording to the teachings of Nelson.